Currently, a secondary cell, such as a nickel metal hydride cell, a lithium ion cell, and a lead acid cell, is used as a backup power supply such as an uninterruptible power system (UPS), so that computers and the like can operate by the discharged energy from the secondary cell when an unexpected event such as a power failure occurs.
Such backup power supply should supply power to computers and the like even when an unexpected event occurs. In terms of a maintenance check, it is important to know the right timing to replace the secondary, and hence it is desired to accurately detect the degradation state of the secondary cell.
However, it is known that the degradation of a secondary cell is complicated and involves various factors such as the temperature, the discharge capacity, the depth of discharge, the elapsed time, the number of discharge times, and the like. It is also known that it is difficult to accurately detect the degradation state of a secondary cell.
An example of an apparatus for judging the life of a secondary cell that has been proposed is a cell life judging device disclosed in Patent Document 1.
With the cell life judging device disclosed in Patent Document 1, an expected life value is calculated based on load power applied to a secondary cell and an ambient temperature. The number of discharge times of the secondary cell is measured, and a first life reduction amount is calculated based on the measured number of discharge times. A second life reduction amount is calculated based on an average temperature of the secondary cell in a charge/discharge mode or in a standby mode, an elapsed time since installation of the secondary cell, and the ambient temperature. A remaining life is calculated by subtracting the first life reduction amount and the second life reduction amount from the expected life value.
The first life reduction amount L1 and the second life reduction amount L2 can be obtained by the following equations:L1=a×ln(b×N)+c L2=d×D×2(Tm−T0/10) 
where N is the number of discharge times, D is the elapsed time, Tm is the average value of temperatures of the secondary cell in a charge/discharge mode or in a standby mode as measured at certain time intervals, T0 is the ambient temperature when the expected life is calculated, and a, b, c, and d are constants.
Patent Document 1: JP-A No. 2007-263952